1. Field of the Invention
The present invention relates to an optical recording medium and a manufacturing method of the same. More particularly, the invention concerns an optical recording medium which is used in a state (near-field) where the distance between an optical lens and the optical recording medium is approached to each other up to 200 nm or less, and a manufacturing method of the same.
2. Description of the Related Art
As an optical recording medium, e.g. optical disk there are the so-called ROM type optical disk used for reproduction only, whose substrate has formed therein beforehand, for example, a pit, a groove used for tracking, etc., and the so-called RAM type optical disk such as a phase change type optical disk or optical magnetic disk used for recording and reproduction, whose substrate having formed therein, for example, the pit, the groove used for tracking, etc. has formed thereon a film such as information-recording layer, etc.
In the phase change type optical disk, by irradiation of laser light, its recording layer is phase changed from an amorphous state of low reflectance to a crystalline state of high reflectance or vice versa, whereby recording of information is performed and, by optically detecting the change in the reflectance, reproduction of information is performed.
Also, in the magneto-optic disk, by application of an external magnetic field or irradiation of laser light, the magnetized state of the recording layer is changed and recording of information thereinto is thereby performed. By this magnetization, the angle of polarization of the reproduction laser light is rotated due to the magneto-optic effect, e.g. Kerr effect resulting from the state of magnetization. By detecting this rotation, reproduction of the information is performed.
Each of those optical disks has a substrate capable of transmitting light therethrough and, on this substrate, has formed-film layers such as a recording layer, a reflection layer, a dielectric protection layer, etc. By a laser light being incident upon the optical disk from the side of the substrate, recording and reproduction are performed with respect to the recording layer.
The recording density of the optical disk generally depends upon the diameter of the laser spot of the light source. The smaller the diameter of the laser spot is, the higher the recording density can be made. The diameter of the laser spot is proportionate to lλ/NA (λ: the wavelength of the laser light and NA: the numerical aperture of the objective lens). Accordingly, in order to increase the recording density of the optical recording medium, it is demanded to make the wavelength of the laser light shorter and make the value of the NA greater.
As the method of realizing the increase in the value of NA, there has in recent years been proposed from Telaster Company a near-field optical disk in which the distance between the optical disk and the optical lens is 200 nm or less. Also, from Quinta Company, there has been proposed an optical hard disk wherein an optical lens is loaded on a slider and the distance between the optical disk and the slider thereby becomes 100 nm or less, etc. In the optical system performing recording or reproduction of each of these optical disks there is contained a solid immersion lens (SIL), and thereby it is possible to obtain a value of NA>1.
By the way, in case the optical system has had its NA increased, there occurs the problem that the coma-aberation becomes large. The coma-aberation is proportionate to (skew angle)×(NA)3×(the distance over which laser light transmits through the optical disk) (provided, however, that the skew angle is an angle of inclination defined with respect to the optical axis of the optical disk). As stated above, because laser light is irradiated onto the recording layer from the side of the substrate, in order to decrease the coma-aberation, it is necessary to make the substrate thin. In view of the fact that a plastic injection molded substrate has hitherto been widely used as the substrate of the optical disk, thinning the substrate with a high precision is difficult in terms of the manufacture.
In contrast to this, there is a recording/reproducing method wherein irradiation of laser light at the time of the recording or reproduction is performed from the side having formed thereon the recording layer of the optical disk so as to largely reduce the distance over which laser light transmits through the optical disk. The use of this method enables decreasing the coma-aberation, and therefore this method becomes suitable for the increase in the value of NA.
As the optical disk there is also an optical disk which, for example, has formed therein grooves for tracking, with an information recording layer being formed in a groove or information recording layer formed on the so-called “land” between adjacent grooves, wherein information is recorded thereon. Further, as the optical disk there is also a land/groove recording type optical disk wherein recording is performed in both each of the grooves and each of the lands. In case recording is performed using this system of land/groove recording, it is possible to increase the recording density.
In the near-field optical disk apparatus wherein as stated above the optical disk and the optical system such as an optical lens are disposed near to each other up to a distance of approximately 200 nm or less, in case there exists a defect which is shaped like a convexity (hereinafter referred to as “a projection”) on the surface of the optical disk, this causes damage to the optical system. Accordingly, the optical disk which is used as a near-field one is needed to have its surface flattened with a high precision. Especially, it is strictly demanded to have no projection on its surface.
Also, in the optical disk made with a method wherein laser light is irradiated from the side having the recording layer, there is a disk of a type having a protection layer whose thickness is approximately 100 μm or so formed on the surface of its recording-layer side. This protection layer is formed using a method of bonding together spin-coats or films of, for example, ultraviolet-ray hardenable resin.
However, in case forming the protection layer made of the ultraviolet-ray hardenable resin, at the time of forming the spin-coat, etc. there occurs therein a swell due to inclusion therein of air bubbles or particles due to the air being entrapped. This swell has no method to eliminate under the existing circumstances. Therefore, the swell becomes a projection on the surface of the optical disk. In other words, the ordinary organic material layer which is made of ultraviolet-ray hardenable resin or the like has high flexibility and therefore after forming the film it is impossible to perform polishing such as, for example, FTP (Flying Tape Polish).
Accordingly, the optical disk having the above-described protection film on its surface is inadequate as the optical disk which is applied to a recording/reproduction apparatus using the near-field optical system wherein the optical disk and the optical system are disposed near to each other up to the distance of approximately 200 nm or less.
However, in case avoiding the formation of the protection layer in order to eliminate such an inconvenience, the recording layer having a concavity and convexity surface on the substrate which has a fine concavity and convexity surface due to the above-described pits, grooves, etc., the recording layer being one having formed thereon the concavity and convexity surface in such a way as for it to follow the configuration of which fine concavity and convexity surface, i.e. in such a way as to reflect this surface, does not have its surface concavities and convexities mitigated by the protection layer. Therefore, especially, in case the resulting optical disk is used together with the near-field optical system wherein the distance thereof from the optical system is approximately 200 nm or less, the distance between the land portion and the groove portion becomes great. This causes the occurrence of the inconvenience that the characteristic decreases, the recording density cannot sufficiently be increased, etc.
Also, by forming a material layer having an appropriate index of refraction on the surface of the optical disk, it is possible to decrease the surface reflection from a prescribed layer or to control multiplex interference with a prescribed layer. Therefore, it is possible to improve the MTF (Modulation Transfer Function). However, in case avoiding the formation of a surface layer such as the protection layer on the surface of the optical disk, the improvement of the MTF cannot be expected.
As stated above, in order to make up the optical disk into a structure of land/groove for increasing the recording density of the optical disk and also to improve the MTF, it is necessary to form a light transmission layer such as a protection layer capable of satisfying prescribed optical conditions on the surface on the laser light irradiation side of the optical disk, i.e. the recording layer side thereof. In this case, the surface should be flattened with a high precision.
In case of forming such a protection layer on the surface of the optical disk in a state of vacuum by the use of a method such as sputtering, the protection layer is also formed within the groove as well with a thickness substantially equal to that of the protection layer on the land. Resultantly, the surface of the protection layer has a configuration that reflects the difference in level of the backing layer. Accordingly, it is necessary to polish the entire surface and flattening the same requires the use of a significantly large length of time. Also, it is also considered as being available to adopt a method wherein the entire surface is polished after depositing a layer of an excessively large thickness, e.g. a protection layer having a thickness of 1 to 2 μm so that the difference in level of the surface may be dissolved. In this case as well, a large length of time is needed. In either case, it is necessary to control the polishing with a high precision so as to make the thickness of the layer uniform.
If forming the protection layer using ultraviolet-ray hardenable resin as the material and by the use of, for example, a spin-coating method, the flat surface is obtained. However, because there is the problem of projections due to inclusion of the air bubbles, etc. and further these projections cannot be abated by polishing as stated above, the decrease in the yield follows.
Further, the formation of the respective layers such as reflection layer, recording layer, dielectric layer, etc. onto the substrate of the optical recording medium is generally performed in an atmosphere of vacuum by sputtering respective materials therefor. In this case, when performing the layer formation, there is a case where due to the surface condition of the sputter surface, the configuration of it, and other reasons abnormal electric discharge occurs during the sputtering. Resultantly, thorn-like projections occur from the surface of the formed layer. And, once these thorn-like projections have occurred, the thereafter-succeeding layer formation performed by sputtering is impossible to flatten the projections. In addition, there is also a case where the height of the thorn-like projections rises even to around the thickness of the formed layer.
And, when those projections have been formed, even if depositing, for example, the protection layer on the layer having the projections thereon, selecting the thickness of that protection layer to have a value which is sufficiently larger than that corresponding to the height of the projection is necessary for obtaining a sufficient flatness on the surface of the protection layer.